The purpose is so that no position is significantly more valuable than any other. As these guys said, a QB can easily get 2-3 TDs in one game, where if a RB gets 1 TD thats a very good game. Also, a QB can get as many as 300-400 yds in a game, where 100 yds is a very good game for a RB. Most leagues make passing yds worth around half as much as rushing or receiving yds.
Props to sigmaker for the sig
"It's not whether you get knocked down, it's whether you get up." - Vince Lombardi